Both of these photos show the famous Golden Gate Bridge near San Francisco, California. The pictures were taken from about the same point of view on the same day, but they look very different. In the picture on the left, the deck of the bridge is almost completely hidden by a thick layer of fog. In the picture on the right the fog has disappeared, and the deck of the bridge—as well as the water below it—is clearly visible. Fog consists of tiny droplets of liquid water. The fog in the picture is like a cloud at ground level. Where did the fog come from, and where did it go?

What Are Changes of State?

The water droplets of fog form from water vapor in the air. Fog disappears when the water droplets change back to water vapor. These changes are examples of changes of state. A change of state occurs whenever matter changes from one state to another. Common states of matter on Earth are solid, liquid, and gas. Matter may change back and forth between any two of these states.

Changes of state are physical changes in matter. They are reversible changes that do not change matter’s chemical makeup or chemical properties. For example, when fog changes to water vapor, it is still water and can change back to liquid water again.

Suppose that you leave some squares of chocolate candy in the hot sun. A couple of hours later, you notice that the chocolate has turned into a puddle like the one pictured in the Figurebelow.

Q: What happened to the chocolate?

A: The chocolate melted. It changed from a solid to a liquid.

In order for solid chocolate to melt and change to a liquid, the particles of chocolate must gain energy. The chocolate pictured in the Figureabove gained energy from sunlight. Energy is the ability to cause changes in matter, and it is always involved in changes of state. When matter changes from one state to another, it either absorbs energy—as when chocolate melts—or loses energy. For example, if you were to place the melted chocolate in a refrigerator, it would lose energy to the cold air inside the refrigerator. As a result, the liquid chocolate would change to a solid again.

Q: Why is energy always involved in changes of state?

A: The energy of particles of matter determines the matter's state. Particles of a gas have more energy than particles of a liquid, and particles of a liquid have more energy than particles of a solid. Therefore, in order for matter to change from a solid to a liquid or from a liquid to a gas, particles of matter must absorb energy. In order for matter to change from a gas to a liquid or from a liquid to a solid, particles of matter must lose energy. You can learn more about the role of energy in changes of state at this URL: http://hogan.chem.lsu.edu/matter/chap26/animate3/an26_035.mov

Summary

A change of state occurs whenever matter changes from one state to another. Changes of state are physical changes in matter. They are reversible changes that do not change matter’s chemical makeup or chemical properties.